Very long optical fiber transmission paths, such as those employed in undersea or transcontinental terrestrial lightwave transmission systems which employ optical amplifier repeaters, are subject to decreased performance due to signal fading and/or fluctuations in the signal-to-noise ratio ("SNR") which are primarily caused by polarization dependent effects. In addition, these lightwave transmission systems are susceptible to degraded performance caused by nonlinearities in the optical transmission fibers.
In a long lightwave transmission system employing amplifiers, the SNR can fluctuate in a random manner. This fluctuation contributes to a phenomenon known as signal fading. Signal fading results in an increased bit error ratio ("BER") for digital signals carried by the transmission system. When the SNR of a digital signal within such a lightwave transmission system becomes unacceptably small which results in an undesirable high BER, a signal fade is said to have occurred. Experimental evidence has shown that signal fading, and the underlying SNR fluctuations, are caused by a number of polarization dependent effects induced by the optical fiber itself and other optical components (e.g., repeaters, amplifiers, etc.) along the long optical fiber transmission path. In particular, polarization dependent BER over long optical fiber transmission paths can be attributed to polarization dependent loss ("PDL"), polarization dependent gain ("PDG"), polarization mode dispersion ("PMD"), and polarization dependent hole-burning ("PDHB"). All of these effects impact signal transmission as a function of the particular state of polarization ("SOP") of an optical signal being propagated along the long optical fiber transmission path.
Fiber nonlinearities can degrade SNR by enhancing optical noise, or by causing distortion in the transmitted optical waveform. These nonlinear interactions increase as a function of the optical power level, and are dependent upon the relative SOP between the signals and the noise. If optical fibers offered a truly linear transmission medium, system performance, as measured by SNR, would improve as optical power was increased. However, the slight nonlinearity of optical fibers places an upper bound upon the level of optical power that can be transmitted thereby limiting the performance of any transmission system employing the fibers.
A prior solution to the problem of SNR fading is to simultaneously launch two signals of different wavelengths and substantially orthogonal relative polarizations into the same transmission path. Since the two signals are launched with equal power and orthogonal SOPs, the overall transmitted signal is essentially unpolarized. This has the advantage of reducing the deleterious effects of the transmission fiber's nonlinear signal/noise interactions, and signal decay caused by PDHB. The average SNR performance improvement with such a transmitter can be substantial, however, such a system is still subject to fading. For example, PMD can alter the orthogonality condition of the two waves, thus re-polarizing the signal. It is well known that the effects of PMD also vary with time. Thus, even the two wavelength source would be subject to SNR fading.
Another approach to reduce SNR fading is to control the launch polarization of a conventional single wavelength transmitter. This has been effective in repeaterless transmission arrangements where signal degradation is primariliy due to PMD. However, adjusting the polarization of a single wavelength at the transmitter only changes the launch state of the signal, and does not take into account the evolution of the axes of polarization of the signal as it propagates along the transmission system. Therefore, single wavelength transmitters cannot facilitate recovery from all SNR fading conditions. In addition, such single-wavelength transmitters cannot reduce performance limitations due to fiber nonlinearities.